Over the last few years, there has been a tremendous growth in the use of multiple-beam antenna (MBA) systems for satellite communications. For example, MBAs are currently being used for direct-broadcast satellites (DBS), personal communication satellites (PCS), military communication satellites, and high-speed Internet applications. These antennas provide mostly contiguous coverage over a specified field of view on Earth by using high-gain multiple spot beams for downlink (satellite-to-ground) and uplink (ground-to-satellite) coverage.
It is known to provide MBA systems having multiple reflectors, each of which supports both transmission and reception of signals. Such systems require a plurality of feed horns for feeding each of the reflectors. The feed horns are designed for providing signal transmission and reception over widely separated respective transmission and reception frequency bands.
For each individual reflector, feed horn efficiency and directivity limits the effectiveness of the antenna system. In particular, an inadequately directive feed horn causes an energy spill over the reflector that can account for up to a 3 dB gain loss, and can also affect pattern performance on the ground.
As shown in FIG. 1, a conflicting set of requirements governs the design of known MBA reflector systems 10. Feed horns 12A, 12B, 12C, 12D feed respective signal beams 14A, 14B, 14C, 14D to the reflector 16. The size of each feed horn 12A, 12B, 12C, 12D limits the angular spacing 13 between each of the respective signal beams 14A, 14B, 14C, 14D. A larger horn 12A, 12B, 12C, 12D having a larger horn aperture improves the efficiency of the MBA reflector system 10 for a given reflector size by decreasing the spillover loss and by increasing the Equivalent Isotropically Radiated Power, or EIRP for transmit satellite antennas (a measurement of power density on the ground), and increases the gain over temperature, or G/T for receive satellite antennas. However, the larger horn 12A, 12B, 12C, 12D having an increased horn aperture also increases the angle β between the respective signal beams 14A, 14B, 14C, 14D, resulting in widely spaced spot beams 18A, 18B, 18C, 18D that produce coverage over a small portion of the overall coverage area. Coverage of any spaces between the widely spaced beams 18A, 18B, 18C, 18D requires the use of additional reflectors 16 to achieve an interleaved beam layout on the ground, increasing cost, complexity and payload requirements of the system.
Typically, gain enhancement from multiple beam reflector antennas can be achieved by increasing the horn gain, reflector shaping, creating an overlapping subarray using a plurality of horns combined via a complex beamforming network, or increasing the number of reflector antennas, sometimes as much as quadruple the number of reflectors.
Gain enhancement lenses are beginning to be used to enhance feed horn gain by improving the effective feed horn aperture. For example, Luneberg lenses having graded indices of refraction using a regular dielectric are well known, but are typically large, heavy, and have a high cost, and are therefore impractical for space applications. Additionally, an elemental gain enhancement lens has been demonstrated based on a thin electromagnetic band gap (EBG) lens. The EBG lens is known to reduce cross-polarization and increases the gain of a small aperture horn antenna array feed system to produce a system of overlapping beams. However, the EBG lens has been demonstrated only over a very narrow (1%-2%) bandwidth. Widely separated simultaneous transmit and receive bands, such as 12/17 GHz or 20/30 GHz bands, are not supported by the EBG lens. Recently, an active lens design having amplifiers inside the lens has been proposed for transmit MBAs. The active lens design concept accepts a high feed-lens spillover loss since this it occurs on the low power side of the high power amplifiers. However, the active lens design concept is in a preliminary stage, and in any event, is only applicable to transmit MBAs.
There is therefore a need for a multi-beam, multi-band antenna with closely spaced antenna feed horns having an increased effective feed horn aperture and a reduced spill over loss that is also capable of simultaneous operation over widely separated transmit and receive bands.